1. Field of the Invention
The present invention relates to an organic electroluminescent element used in a display or the like, a production method of the same, and an organic electroluminescent display. More particularly, the present invention relates to an organic electroluminescent element in which a change of a shade is controlled, a production method of the same, and an organic electroluminescent display.
2. Description of the Related Art
In an organic electroluminescent (EL) device, a layer formed by doping a guest material (dopant) to a host material in an aimed concentration has been used as a luminescent layer to obtain a practical level of chromaticity. FIG. 1 is a cross-sectional view showing a conventional organic EL element.
In the conventional organic EL element, on a glass substrate 11 is formed a transparent electrode composed of ITO (Indium-Tin-Oxide) as an anode 12. Further, on the anode 12 is formed an organic EL layer composed of a hole transport layer 13 which transports a hole, a luminescent layer 14 which emits light by a recombination between a hole and an electron, and an electron transport layer 15 which transports an electron. Further, on the electron transport layer 15 is formed a cathode 16. A guest material is doped into the luminescent layer 14 in a predetermined concentration.
In the thus formed conventional organic EL element, a hole or an electron penetrates the luminescent layer due to the relation of an applied voltage so that a change of carrier balance can possibly occur. Therefore, a recombination between a hole and an electron occurs in the electron transport layer or the hole transport layer so that the electron transport layer or the hole transport layer emits light, whereby a shade is undesirably changed.
Japanese Patent Application Laid-Open No. Hei 8-315982 discloses an organic EL element that can generate a plurality of color tones in accordance with an applied voltage to prevent a change of a shade. In the organic EL device described in this publication, an aluminum complex is used in the luminescent layer or the electron transport layer. And the electron transport layer (or fluorescent dyes doped into the electron transport layer) is light-emitted at a low applied voltage, on the other hand, the hole transport layer (or fluorescent dyes doped into the hole transport layer) is light-emitted at a high applied voltage.
Further, Japanese Patent Application Laid-Open Nos. Hei 10-231476, 10-231477, and 10-231478 disclose an organic EL element provided with a hole blocking layer between an organic luminescent layer and a cathode. In the organic EL element described in these publications, a triazole compound is used as a hole blocking material. Further, these publications describe that dopants are doped in the hole transport layer to obtain a higher luminescence efficiency and an excellent heat resistance.
However, the conventional organic EL element described in Japanese Patent Application Laid-Open No. Hei 8-315982 has a problem in that when luminescence is performed in the electron transport layer or hole transport layer the emitted colors are not the same as those of the luminescent layer.
Further, even if a hole blocking layer is provided to increase the luminescence efficiency like the conventional organic EL element described in Japanese Patent Application Laid-Open No. Hei 10-231476 and the like, holes that penetrate the luminescent layer and are blocked with the blocking layer don""t have a significant ratio that contributes to the luminescence. Accordingly, the conventional organic EL element cannot control a change of a shade. If a dopant was doped in the hole transport layer, there are some cases which a change of a shade can be controlled. However, only doping of the dopant cannot stably control such a change.
As explained above, the luminescence of the electron transport layer or the hole transport layer itself by the hole or electron, which penetrates the luminescent layer, cannot be sufficiently controlled even by these conventional examples.
An object of the present invention is to provide an organic electroluminescent element, which can sufficiently control a change of a shade by luminescence of an electron transport layer or a hole transport layer itself, and can always hold a stable chromaticity, and a production method of the same, and an organic electroluminescent display.
An organic electroluminescent element according to the present invention, comprising a dopant-doped luminescent layer that emits light by a recombination between a hole and an electron, a hole transport layer that transports a hole supplied from an anode to the luminescent layer, and an electron transport layer that transports an electron supplied from a cathode to the luminescent layer, is characterized in that the same kind of dopant as that of the dopant is doped in the region, where at least one of the hole transport layer and the electron transport layer contacts the luminescent layer, in a lower concentration than the concentration in the luminescent layer.
According to the present invention, since the same kind of dopant as that of the dopant in the luminescent layer is doped in the region, where at least one of the hole transport layer and the electron transport layer contacts the luminescent layer, in a lower concentration than the concentration in the luminescent layer, when a recombination between the hole and the electron is generated in at least one of the hole transport layer and the electron transport layer, luminescence is performed in a dopant-doped region. As a result, even if at least one of the electron and the hole penetrates the luminescent layer due to the relation of an applied voltage, the current efficiency is not lowered and the luminescence in the hole transport layer and the electron transport layer is controlled. Thus, a change of a shade is stably controlled.
Note that it is preferable that the thickness of a region where the dopant is doped into at least one of the hole transport layer and the electron transport layer is ⅔ or less of the thickness of one of the hole transport layer and the electron transport layer or of both thickness thereof. This reason is that a too thick doped layer causes reduction in a doping efficiency, and on the other hand, a too thin doped layer causes a small control effect on a change of a shade. Accordingly, by designing the thickness of the doped region to ⅔ or less of the thickness of one of the hole transport layer and the electron transport layer or of both thickness thereof, an excellent doping efficiency and the control effect on a change of a shade can be obtained.
A method of producing an organic electroluminescent element according to the present invention, comprising the steps of forming a hole transport layer that transports a hole supplied from an anode, forming a dopant-doped luminescent layer that emits light by a recombination between a hole transported from the hole transport layer and an electron transported from the hole transport layer, and forming an electron transport layer that transports an electron supplied from a cathode to the luminescent layer, is characterized in that the step of forming at least one of the hole transport layer and the electron transport layer includes a step of doping the same kind of dopant as that of the dopant in a region, where at least one of the hole transport layer and the electron transport layer contacts the luminescent layer, in a lower concentration than the concentration in the luminescent layer.
According to the present invention, since the same kind of dopant as that of the dopant in the luminescent layer is doped in the region, where at least one of the hole transport layer and the electron transport layer contacts the luminescent layer, in a lower concentration than the concentration in the luminescent layer, a change of a shade is stably controlled as described above.
Further, it is preferable that the thickness of the region where the dopant is doped into at least one of the transport layer and the electron transport layer is ⅔ or less of the thickness of one of the hole transport layer and the electron transport layer or of both thickness thereof. This reason is that a too thick doped-layer causes reduction in a doping efficiency, and on the other hand, a too thin doped-layer causes a small control effect on a change of a shade. Accordingly, by designing the thickness of the doped region to ⅔ or less of the thickness of one of the hole transport layer and the electron transport layer or of both thickness thereof, an excellent doping efficiency and the control effect on a change of a shade can be obtained.
Further, an organic electroluminescent display according to the present invention is characterized in that the above-mentioned organic electroluminescent element is mounted.